All wheel steering system

ABSTRACT

An all wheel steering system for heavy vehicles such as airport and municipal service vehicles. The system includes several display devices to inform the operator of the operational status of various components of the system. A rear wheel lock-out is provided to ensure safe operation by preventing rear steering above a pre-set vehicle speed. The system can be adjusted so that rear steering response timing and front and rear steering range angles can be varied to fit particular applications.

This application is a continuation of application Ser. No. 390,728,filed Aug. 8, 1989 now abandoned.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a steering system for heavy vehicles in whichtight cornering and a high degree of maneuverability are importantfeatures. In particular, the invention relates to a steering system inwhich the front and rear wheels of a vehicle can be operated in avariety of modes, including front only, coordinated (or radiarc), crab,and rear/front independent.

Systems for controlling the steering of vehicles have includedmechanical systems like the one shown in U.S. Pat. No. 3,903,983, inwhich cables and related mechanisms are arranged so that a rear steeringlever is made to mechanically follow on depend upon front wheelsteering. U.S. Pat. No. 4,175,638 shows an electronically controlledfour wheel steering system in which electronic controls for selectingsteering modes and for independently controlling rear wheel steering aremounted adjacent to one another on a lever or control panel.

The steering systems discussed above, and other prior art steeringsystems generally, have lacked several important features relating tosafety, reliability and versatility. Clearly displaying the status ofvarious aspects of the system has been limited. Complete and exactstatus information is important for safe operation. Examples of vehicleswhere four wheel steering is used, both on and off road, are runwaysweeping machines, tow-tractors, refueling vehicles, crash/fire rescuevehicles, snow-plowing and blower vehicles, and military on-off roadall-wheel drive vehicles.

It is an object of the present invention to provide a steering systemwhich is efficient and which provides the operator with a variety ofimportant system status information.

Another object of the invention is to provide a steering system whichcan be safely operated in a variety of modes.

Another object of the invention is to provide a steering system whichlimits the speed at which the vehicle can be operated while in aparticular mode.

Another object is to provide a steering system in which various controlscan be adjusted to meet the needs of a particular user or a particularapplication.

A further object is to provide a steering system in which modes may bechanged while the vehicle is in motion.

Yet another object of the present invention is to provide a steeringsystem in which the angle of disposition of the rear wheels isgraphically displayed in order to assist the operator.

These and other objects and advantages of the invention are obtainedwith a steering system which has front end and rear sensors whichgenerate a signal corresponding to the position of the wheels of avehicle. The sensors are part of a circuit which is used to control thesteering system. The control unit can be set to a variety of steeringmodes so that the vehicle can be operated in at least four differentways, namely, "front only", "coordinated" (rear wheel steering movementopposite to that of the front), "crab" (rear and front move the sameway), and "independent" (rear and front wheels are independentlycontrolled). The system includes a rear steering lockout which iscomprised of a pin which prevents steering of the rear wheels. Thelockout may be set to activate at a predetermined vehicle speed. Thesystem also includes a way of adjusting the range of steering anglesthrough which the wheels are allowed to move.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be better understood upona reading of the following specification, read in conjunction with thefollowing drawings wherein:

FIG. 1 is a series of four schematic diagrams which show the modes inwhich the steering system of the present invention may be operated; and

FIG. 2 is a schematic diagram of a vehicle showing the generalarrangement of the controls and operational elements of the system ofthe present invention; and

FIGS. 3 through 5 show display and switching panels which are used withthe present invention; and

FIG. 6 is a schematic diagram of the circuit used to control thesteering system of the present invention.

FIG. 7 is a schematic diagram of the circuit used to deliver power tothe circuit shown in FIG. 6.

FIG. 8 is a schematic diagram of the circuit used to control a safetyfeature of the system of the present invention.

FIG. 9 is a schematic diagram of a DC to DC converter used to supplyvoltage to various points in the circuit of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

The four wheel steering system of the present invention has thecapability of operating in the four modes shown in FIG. 1. In the "frontonly" mode 12, the rear wheels are held in the center position, and thefront wheels may be steered with a standard steering wheel. In the"coordinated" mode 14, the movement of the rear wheels 28 is controlledby operation of the front wheels. However, movement of the rear wheelsis opposite in direction relative to movement of the front wheels. Inthe "crab" mode 16, movement of the rear wheels is dependent on and inthe same direction as movement of the front wheels. Finally, in the"independent" mode 18, movement of the front wheels is controlled with asteering wheel, while movement of the rear wheels is controlled byoperation of a joystick (see FIGS. 3 and 6).

FIG. 2 shows a plan schematic view of several of the mechanicalcomponents of the steering system of the present invention. Front axle20 carries front wheels 22, the orientation of which is monitored byangle sensor 24. Similarly, rear axle 26 carries rear wheels 28, theangular orientation of which is monitored by angle sensor 30. The rearaxle lock-out 32, when activated, prevents steering motion of the rearwheels. Front steering mechanism 33 and rear steering mechanism 34 areeach hydraulically operated by a standard steering pump 35 and a loadsensing rear steering pump 37. The steering system further includes ahydraulic reservoir 41, a return filter 43 and a pressure filter 45, asolenoid valve 47, and a priority flow regulator 29. The control unit36, which may be located in the cab of the vehicle, is a centralswitching point for several of the various components of the steeringsystem of the present invention. As shown in FIG. 2, those componentsinclude an electro-hydraulic valve 39, mode switches 62, speed switch88, speed switch reset 60, lock out indicator 40, priority flow solenoidvalve 47, lockout mechanism 32, and diagnostic indicators 59.

FIGS. 3, 4 and 5 show some of the displays and controls which are partof the steering system. FIG. 3 shows a lock-out indicator light 40which, when lit, indicates that the rear axle lock-out 32 is disengaged.When the light 40 is not lit, the rear axle lock-out is engaged. Thedisplay 40 is important because steering of the rear wheels of a vehicleat even moderate speeds can be dangerous. As will be explained later,the speed at which the lock-out is automatically engaged should normallybe set at 20 m.p.h. However, the speed at which lock-out occurs can beadjusted in the steering system of the present invention. Also shown inFIG. 3 is the rear steering control, which is preferably a joystick 42,which operates between an extreme left position 44 and an extreme rightposition 46. The joystick locks into its center detented position. Alift tab (not shown) is required to be pulled up in order to unlock thejoystick. The joystick is of the friction-hold type, so any positionfrom center can be set and held without the operator's hand being on theunit. While in the rear mode, positioning the joystick 42 in the centerposition causes the rear wheels to be centered. In other systems whichemploy a momentary rocker switch, the rear wheels must be adjusted tocenter without clear information as to when centering has occurred.

FIG. 4 shows another display in which a light emitting diode (LED) bargraph 48 shows the angular position of the rear wheels. The lightemitting diode (LED) graph 48 operates in all modes to indicate theposition of the rear tires. When the joystick 42 is moved to theposition 46, the entire left half of the bar graph 48 is lit. When thejoystick is moved to an intermediate position between the center and theextreme positions, there will be a lit portion, extending from thecenter, which corresponds to the angle of the rear wheels.

In addition, FIG. 4 shows a series 59 of indicator lights. The indicatorlight 50 indicates that power to the steering system is on. Theindicator light 52 indicates that the lock-out pin is engaged. It shouldbe noted that the lock-out indicator 40 discussed above indicates thatthe lock-out mechanism is unlocked, whereas the indicator light 52indicates that the lock-out pin is, in fact, engaged. Light 40 goes outat approximately the same time that light 52 goes on. The light 40 goingoff is a positive indicator of the rear axle being locked. The indicatorlights 54 and 56 indicate that power is being supplied to the left andright sides of the joystick potentiometer. Finally, the indicator light58 shows whether power is being delivered to the wheel angle sensors 24and 30. Additional indicating LEDs are located on the integratedproportional amplifiers 120 and 122 indicating the amplifier status, andon the speed switch 88, indicating presence or absence of the pulsegenerator signal.

FIG. 5 shows two important switches which are part of the steeringcontrol of the present invention. The reset switch 60 is a momentarytoggle switch which, when thrown, causes the lock-out 32 to disengagethe rear axle in order to allow all-wheel steering. Reset will onlydisengage the rear axle when two conditions are met, i.e. 1) the vehiclespeed is under 20 mph, and 2) the joystick is in the center position.Finally, the rotary switch 62 determines the mode in which the systemoperates. The four positions correspond to the four modes discussed inFIG. 1.

The main steering circuit shown generally in FIG. 6 operates as a closedloop position feedback system. The angle sensors 24 and 30 generatecommand and feedback signals 74 and 76, respectively, which areprocessed by the electronic control unit of the system. For purposes ofclarity, the corresponding switching portions and activation coilportions of the relays shown in the circuit have been separated on thedrawings, but have been given the same reference numerals.

The front wheel sensor 24 and the rear wheel sensor 30 are rotaryvariable inductance transducers (RVIT), each of which provides adifferential output voltage [±2 volts DC per ±40 degrees] proportionalto the degree of steering of the respective front and rear wheels. Asthe front wheels are steered from left to right, the voltage at theoutput of the RVIT 24 changes from positive to negative in proportion tothe steering angle. This becomes the position command signal. The RVIT30 produces the voltage which provides the position feedback signal.This system is a null-seeking system. Therefore, the polarity of theinput signal must be opposite in polarity, but equal in magnitude, tothe feedback signal for the system to reach equilibrium, causing thevalve to center. The two voltages are amplified by the instrumentationamplifiers 80 and 82, and are summed together by precision operationalamplifier 112, and any difference between these two voltages is passedon as the rear steering signal 114 to the integrated proportionalamplifiers 120 and 122. These amplifiers 120 and 122 process the rearsteering signal 114 from the summing amplifier 112, and drive theelectro-hydraulic proportional valve 39 to properly orient the rearwheels.

As shown in FIG. 6, the mode switch 62 lies between the front RVIT 24and a command signal instrumentation amplifier 80. Similarly, the rearRVIT 30 produces a signal which is processed by a second instrumentationamplifier 82. When the mode switch 62 is set for the front steering mode12, the signal from the front RVIT is cutout and set to a false zerosignal. Since the RVITs are mounted such that a zero volt output willoccur when the wheels are straight ahead, and the system isnull-seeking, this false zero will cause the electro-hydraulicproportional valve to position the rear wheels straight ahead. Thepolarity of the command RVIT 24 must be properly phased relative to thefeedback RVIT 30 in order to provide the "coordinated" mode 14. Sincethe system is null-seeking, the input voltage must be opposite thepolarity of the feedback signal. For the "coordinated" mode, thiscondition must be met as the wheels are going in the oppositedirections. By switching the polarity of the front wheel RVIT 24, we cancontrol the direction of movement of the rear wheels in relation to thefront wheels. Thus, we can alternate from "crab" mode 16 to"coordinated" mode 14.

Finally, in the "independent" mode 18, the joystick 42 is activatedthrough the relay 84. Changes in realtive polarity of the command andfeedback signals should be done by varying the polarity of the frontRVIT 24 so that a constant phase relationship may be maintained betweenthe rear RVIT 30 and the electro-hydraulic valve 39.

The indicator lights or LEDs 54 and 56 indicate power to the left andright sides of the joystick potentiometer, respectively. The joystick 42is equipped with a switch 86, which is part of the subcircuit shown inFIG. 8. The switch 86 is activated by centering of the joystick. Thesubcircuit of FIG. 8 controls the activation and resetting of theair-operated rear wheel lock out mechanism 32. The center switch 86 mustbe activated in order to allow the reset switch 60 to be activated. Thevariable speed switch 88, together with the pulse generator 89, triggersthe engagement of the rear wheel lock-out at a predetermined vehiclevelocity. When the vehicle reaches the predetermined velocity, the relay90 is de-activated, causing the air-operated lock-out pin mechanism 32to engage the rear axle 26 when the driver re-centers the rear tires.Reverse Relay Logic is used. The pin engages without power and unlockswith power, thus providing a fail safe mode if power is lost.

Referring again to FIG. 6, a microswitch 92 is triggered by engagementof the pin of the lock-out mechanism 32. When the switch 92 is thrown,the lock switch LED 52 is lit, and the lock-out indicator 40 goes off.When the lock switch LED 52 goes on, the relay 94 is also activated.Activation of the relay 94 has two consequences. First, the commandsignal is grounded, making the rear wheels stay at the center position.Second, current is prevented form flowing through the timing oscillator98. When the lock-out pin 32 is removed and switch 92 returns to itsnormally open position relay 94 is deactivated, at which time the lowerportion 94a of the relay 94 completes part of the command circuit, andthe upper portion 94b of the relay 94 activates the timing oscillator 98and relay 96. For the adjustable time delay, relay 96 will be activatedto send ground as the command signal, thus holding the rear wheelsstraight ahead for a few seconds. This occurs when relay 94 isde-activated and +12 volts are fed to the timing oscillator. The delayof the completion of the command circuit may be adjusted by varying thepotentiometer 100. The delay of the timing oscillator is to prevent anysteering of the rear axle prior to complete removal of the pin of thelock-out mechanism 32. If steering is allowed to occur immediately uponclosing of the switch 92, jamming or shearing of the pin could occur.

One part of the feedback signal 76 is processed by the uniquecombination of two operational amplifiers 106 and 107 shown in FIG. 6.When signal 76 is on an negative swing, it will be converted to apositive voltage and LED driver 128 will drive one half of the LED bargraph display. On a positive swing, this voltage will be converted tozero, thus LED driver 128 will be off.

The other part of the feedback signal 76 is fed directly to LED driver130. On a positive swing, this voltage will cause LED driver 130 todrive the other half of the LED bar graph display in the otherdirection. On a negative swing, LED driver 130 will be off.

Thus the LEDs sweep from center in the direction that the rear wheelsare turning. No LEDs on means that the rear tires are centered.

The adjustability of the steering system of the present invention is animportant aspect. By adjusting the potentiometers 102 and 104, which areconnected to the instrumentation amplifiers 80 and 82, the range ofsteering angles of the front and rear wheels, respectively, can beadjusted. For example, in the "coordinated" and "crab" modes, the rangeangle of the rear wheels can be made to match the range angle of thefront wheels, approximately 28° to 32°. In addition, the range angle ofthe rear wheels can be modified by adjusting the potentiometers 103 and105 so that in the rear steering mode the rear range angle can be eitherdifferent from or the same as the range angle as determined by thesettings of potentiometers 102 and 104. As used in this specification,the term "range angle" refers to the included angle between a steeredwheel in its center position and its full right or left position.

It should also be noted that by adjusting the priority flow regulator29, the speed at which the rear wheels respond to the joystick control42 may be adjusted. When the mode selector is in the rear mode 18 relay124 (See FIG. 6) is activated, which in turn activates a 3-way solenoidvalve 47. This valve directs the hydraulic flow through the priorityflow regulator 29, which in turn regulates the amount of flow to therear steering system. Thus, determining the reaction time of rear wheelsteering in the rear mode 18. In the "coordinated" and "crab" modes, thespeed at which the rear wheels move through the range angle should beapproximately the same as the speed at which the front wheels movethrough their range angle. When in the "independent" mode, fast changesof the rear steering angle can be dangerous. Therefore, the steeringspeed of the rear wheels in the "independent" mode should beapproximately half of the speed at which the rear wheels move in the"coordinated" and "crab" modes.

The speed of steering the rear wheels can be adjusted by varying thepotentiometers located on the integrated proportional amplifiers 120 and122. The valve deadband can be eliminated by potentiometer adjustmentshere also.

In FIG. 7, the subcircuit 70 shows the power coming from the vehicle tothe steering system, and the LED 50 discussed above is included in thatsubcircuit. The parking brake stop light switch 72 must be released inorder for power to reach the steering system. The subcircuit 70 includesa transient overvoltage suppressor 71 and self resetting circuit breaker73. FIG. 9 shows a DC to DC converter 77, which is used to convert the 5volt supply of subcircuit 70 to a positive and negative 9 volt powersource.

The invention has been described with respect to a particularembodiment. However, there are numerous alternatives, modifications andvariations which will occur to those skilled in the art. All suchalternatives, modifications and variations are intended to fall withinthe scope of the appended claims.

I claim:
 1. A four wheel steering system for a vehicle comprising front and rear pairs of steerable wheels, control means for selecting a plurality of steering modes, said modes including a first mode in which steering of said rear pair of wheels is generally fixed, a second mode in which movement of said rear pair of wheels is generally opposite to that of said front pair, a third mode in which movement of said rear pair is generally similar to that of said front pair, and a fourth mode in which movement of said rear and front pairs of wheels can be controlled independently and simultaneously from a single steering control location, locking means for limiting steering of said rear pair of wheels, said locking means being automatically actuated when said vehicle exceeds a predetermined vehicle speed.
 2. A steering system in accordance with claim 1 wherein:said control means includes means for adjusting the predetermined vehicle speed, at which said locking means is activated.
 3. A steering system in accordance with claim 1 wherein:said control means includes lock indicator means for informing an operator as to actual physical engagement and disengagement of said locking means with operating mechanisms which cause movement of said rear pair of wheels.
 4. A steering system in accordance with claim 1 wherein:engagement of said locking means prevents said system from operating in any mode other than said first mode until said locking means is disengaged.
 5. A steering system in accordance with claim 1 including:rear wheel display means for constantly informing an operator as to the orientation of said rear pair of wheels.
 6. A steering system in accordance with claim 5 wherein:said rear wheel display means includes a graphic imaging device, said device producing a display which is proportional to the steering angle of said rear pair of wheels.
 7. A steering system in accordance with claim 1 wherein:movement of said rear pair of wheels when said second and third modes are selected is substantially faster than movement of said rear pair of wheels when said fourth mode is selected, speed of said movements being relatively adjustable to a predetermined front/rear ratio.
 8. A steering system in accordance with claim 7 wherein:said ratio is about 1:1 for said first and second modes and about 2:1 for said fourth mode.
 9. A steering system in accordance with claim 1 wherein:said system includes display means for informing an operator as to operational status of mechanism for causing movement of said pair of rear wheels, and for informing an operator as to operational status of sensors used to detect positions of said front and rear pairs of wheels.
 10. A steering system in accordance with claim 1 wherein:movement of said front pair of wheels between their center positions and their extreme left and right positions defines a front steering range angle, and movement of said rear pair of wheels between their center positions and their extreme left and right positions defines a rear steering range angle, said rear steering range angle being generally equal in magnitude to said front steering range angle when said system is in said second mode and said third mode, said front steering range angle and said rear steering range angle defining a range angle ratio.
 11. A steering system in accordance with claim 10 wherein:said control means includes means for adjusting said range angle ratio.
 12. A four wheel steering system for a vehicle comprising front and rear pairs of steerable wheels, control means for selecting a plurality of steering modes, said modes including a first mode in which steering of said rear pair of wheels is generally fixed, a second mode in which movement of said rear pair of wheels is generally opposite to that of said front pair, a third mode in which movement of said rear pair is generally similar to that of said front pair, and a fourth mode in which movement of said rear and front pairs of wheels can be controlled independently and simultaneously from a signal steering control location, locking means for limiting steering of said rear pair of wheels, said locking means being automatically actuated when said vehicle exceeds a predetermined vehicle speed,said system including rear wheel display mean for constantly informing an operator as to the orientation of said rear pair of wheels.
 13. A steering system in accordance with claim 12 wherein:said control means includes means for adjusting the predetermined vehicle speed, at which said locking means is activated.
 14. A steering system in accordance with claim 12 wherein:said control means includes lock indicator means for informing an operator as to actual physical engagement and disengagement of said locking means with operating mechanisms which cause movement of said rear pair of wheels.
 15. A steering system in accordance with claim 12 wherein:engagement of said locking means prevents said system from operating in any mode other than said first mode until said locking means is disengaged.
 16. A steering system in accordance with claim 12 wherein:said rear wheel display means includes a graphic imaging device, said device producing a display which is proportional to the steering angle of said rear pair of wheels.
 17. A steering system in accordance with claim 12 wherein:movement of said rear pair of wheels when said second and third modes are selected is substantially faster than movement of said rear pair of wheels when said fourth mode is selected, speed of said movements being relatively adjustable to a predetermined front/rear ratio.
 18. A steering system in accordance with claim 17 wherein:said ratio is about 1:1 for said first and second modes and about 2:1 for said fourth mode.
 19. A steering system in accordance with claim 12 wherein:said system includes display means for informing an operator as to operational status of mechanisms for causing movement of said pair of rear wheels, and for informing an operator as to operational status of sensors used to detect positions of said front and rear pairs of wheels.
 20. A steering system in accordance with claim 12 wherein:movement of said front pair of wheels between their center positions and their extreme left and right positions defines a front steering range angle, and movement of said rear pair of wheels between their center positions and their extreme left and right positions defines a rear steering range angle, said rear steering range angle being generally equal in magnitude to said front steering range angle when said system is in said second mode and said third mode, said front steering range angle and said rear steering range angle defining a range angle ratio, said control means including means for adjusting said range angle ratio. 